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Solar System
A solar system is comprised of a star, whereby celestial objects travel around it — such as planets, moons, asteroids, comets and meteoroids. Stars that host their own planets are called Planetary systems. There are likely tens of billions of other systems like these, in the Milky Way galaxy alone. Solar systems having more than one star are called: binary star systems if there are two stars, or multi-star systems if there are three or more stars. Solar System Our Solar System is located in an outer spiral arm of the Milky Way galaxy. It consists of the Sun, a single star, and everything that orbits around it. This includes eight planets and their natural satellites (such as our moon), dwarf planets and their satellites, as well as asteroids, comets and countless particles of smaller debris. Size and Distance The Solar System extends much farther than the eight planets that orbit the sun. It's system includes the Kuiper Belt that lies past Neptune's orbit. This is a sparsely occupied ring of icy bodies, almost all smaller than the Kuiper Belt Object, Pluto. ;Oort Cloud Beyond the fringes of the Kuiper belt is the Oort Cloud. This giant spherical shell surrounds our solar system. It has never been directly observed, but its existence is predicted based on mathematical models and observations of comets that likely originate there. The Oort Cloud is made of icy pieces of space debris the sizes of mountains and sometimes larger, orbiting our sun as far as 1.6 light years away. This shell of material is thick, extending from 5,000 astronomical units to 100,000 astronomical units. One astronomical unit (or AU) is the distance from the sun to Earth, or about 93 million miles (150 million kilometers). The Oort Cloud is the boundary of the sun's gravitational influence, where orbiting objects can turn around and return closer to our sun. ;Heliosphere The sun's heliosphere doesn't extend quite as far. The heliosphere is the bubble created by the solar wind — a stream of electrically charged gas blowing outward from the sun in all directions. The boundary where the solar wind is abruptly slowed by pressure from interstellar gases is called the termination shock. This edge occurs between 80-100 astronomical units. Two NASA spacecraft, launched in 1977, have crossed the termination shock: Voyager 1 in 2004 and Voyager 2 in 2007. But it will be many thousands of years before the two Voyagers exit the Oort Cloud. Formation The Solar System formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. When this dust cloud collapsed, it formed a solar nebula — a spinning, swirling disk of material. ;Accretion At the center, gravity pulled more material in. Eventually the pressure in the core was so great that hydrogen atoms began to combine and form helium, releasing a tremendous amount of energy. Thus, the Sun was born eventually amassing more than 99 percent of the available matter. ;Gravitational pull Matter farther out in the disk was also clumping together. These clumps smashed into one another, forming larger and larger objects. Some of them grew big enough for their gravity to shape them into spheres, becoming planets, dwarf planets and large moons. In other cases, planets did not form: the asteroid belt is made of bits and pieces of the early solar system that could never quite come together into a planet. Other smaller leftover pieces became asteroids, comets, meteoroids, and small, irregular moons. ;Order of bodies The order and arrangement of the planets and other bodies in our solar system is due to the way the solar system formed. Nearest the sun, only rocky material could withstand the heat when the solar system was young. For this reason, the first four planets — Mercury, Venus, Earth and Mars — are terrestrial planets. They're small with solid, rocky surfaces. The materials such as ice, liquid or gas settled in the outer regions of the young solar system. Gravity pulled these materials together, forming the gas giants Jupiter and Saturn and ice giants Uranus and Neptune. Heliocentrism In the early 17th century, Galileo Galilei's discoveries using the recently invented telescope strongly supported the concept of a solar system in which all the planets, including Earth, revolve around a central star — the sun. At the time this was called Copernican heliocentric theory, and it was a revolutionary idea, as most people thought Earth was the center of the universe. Since then, we have learned much about our solar system and what lies beyond it using ground-based telescopes, spacecraft and mathematical models. Fun Facts * Voyager 1 was the first spacecraft to leave our solar system. References * NASA, Our Solar System: In Depth Category:Solar System